I really like trailers and making them, but of course it always depends on the source material.
Simple casual game trailer have more or less only gameplay, cutted in an interesting way.
Here some examples:
My current trailers for casual games of ours:SKIRMISH Trailer
(actually not the final trailer for release...)Black NebulaEssence
(from 2007, one of my first)
If you have a game with dialog and story, you want to have voices and ambiguous quotes and stuff, raise tension over time...
And the problem is footage of course if its not your own game...
For once, if you use 720p FRAPS video, like I do, its Gigabytes of data because its raw rgb video (or almost)
and also, to get the footage, one has to know exactly what to show and how in the game; which is especially critical if the game is not done and you dont want to expose not finished areas/features